Multicast PIM Designated Router

[lagapidis (Lazarus Agapidis)]

Hello Sam

@samirkhair

Would I be right in saying that the PIM DR will still hear all the joins and prunes, just like the querier?

Yes, that is correct. The PIM DR handles PIM control-plane tasks like sending Register messages to the RP and processing multicast traffic forwarding while the IGMP Querier sends periodic IGMP membership queries. But, both routers will hear IGMP joins/prunes since these reports are sent to the well-known multicast address 224.0.0.22 (IGMPv3) or 224.0.0.2 (IGMPv2), which all multicast routers listen to. The DR uses this information to build PIM joins/prunes, while the Querier maintains group membership timers.

@samirkhair

And because of this it seems to flood IGMP Joins out of every port. Does that behaviour sound correct? i.e. if PIM snooping isn’t available/enabled on a switch IGMP Joins/Prunes are flooded everywhere to ensure they reach the PIM DR.

When PIM snooping is disabled, switches cannot intelligently forward PIM control messages and will flood IGMP reports to all ports except the receiver port. Basic IGMP snooping will be used only to track the Querier port (as shown in your show ip igmp snooping mrouter output). This flooding ensures PIM DRs receive necessary join/prune messages despite the switch’s inability to identify PIM routers specifically.

I hope this has been helpful!

Laz